A liquid crystal cell has liquid crystal material between optical substrates. When making a single cell (even one big cell or a 2D array of cells), the liquid is typically side filled in the vacuum, usually by injecting or by capillary action, and then the hole through which the liquid was introduced is sealed. In this way the contact between the liquid crystal and liquid adhesive (before its solidification) is minimized and the region of contact is outside of the working area of the liquid crystal. In modern (e.g., wafer-scale) manufacturing of liquid crystal devices, large arrays of cells are prepared on a common substrate and then are diced into multiple individual units. In this case, side filling is very slow; it generates significant losses of liquid crystal material and creates contamination of the whole structure, requiring, in addition, a post-dicing clean-up and sealing for each unit. So to avoid those problems, in a process known as “drop fill,” the liquid crystal material is added in the open cell on a bottom substrate and then the top substrate is then sealed onto the bottom.
In some prior art techniques the same material is used to form container walls and to serve as adhesive (initially liquid) between top and bottom substrates. The spacing between the substrates is assured by spacer beads, which may be mixed with the adhesive that seals the liquid crystal. However, this arrangement can lead to contamination of the liquid crystal by the unhardened adhesive.
FIG. 1 illustrates a wafer level assembly of liquid crystal devices according to the prior art. The industry is moving towards wafer level assembly (WLA) to reduce the cost of manufacturing. In the state of the art, this is applied to displays. In the present invention, this is also applicable to tunable optical devices, such as lenses, and thus the wafer level assembly can comprise arrays of various components, for example, image sensors, infrared filters, lenses, tunable elements (tunable lens, diaphragms, etc.) that are diced or singulated into individual devices. In the figure, wafer 10 includes multiple, simultaneously fabricated devices 12 that are subsequently singulated along the lines separating them, as is known in the art. A key is provided in the figure to distinguish the die boundary, liquid crystal, spacer wall and sealant.
In U.S. Pat. No. 6,219,126 to von Gutfeld and assigned to IBM, there is disclosed such a drop fill technique for liquid crystal displays in which the liquid crystal is contained within a barrier fillet around which an adhesive is placed. A technique such as this is shown schematically in FIG. 2. In this figure, barrier fillet 14 is a hard material and only a few micrometers high, such as lithographically fabricated, and provides the spacing between top substrate 16 and bottom substrate 18 in addition to preventing, during assembly and curing of the adhesive 20, contamination of the liquid crystal material 22 by the adhesive. In one embodiment, a spillover area between the barrier fillet and the adhesive is provided to receive excess liquid crystal if the drop has a greater volume than the cell.
However, lithography cannot be cost effectively used for thick elements (such as approximately 50 micrometer) and the hard walls impose unacceptable (for practical manufacturing processes) precision requirements.
FIG. 3 illustrates the more common approach of using an uncured adhesive as the barrier retaining the liquid crystal drop. In this example, a drop 24 of working liquid is located between two non-cured reservoir walls 26. A bottom substrate 28 and a movable top substrate 30 may be used to enclose the space between the walls 26 allowing the working liquid to be effectively retained within the space defined for the “optical window.” Electrodes, thin-film transistors and the like can also be used in the thin layers 32 on the substrates as appropriate for the application in question.